The invention relates to a sound deadening system made of foam material.
Sound absorbing materials are, in the present state of the art, open-cell fiber or foam substances whose structure inhibits the alternating movements of the air which are produced by sound and which thus transform sound energy to heat by viscous friction. Upon entering into an open-pored substance of this kind the sound does not pass from one medium into another but remains in the air. Therefore, if the porosity is sufficiently great--and in most sound deadening substances it is greater than 95%--it can easily penetrate into the substance.
The requirement that sound deadening substances have open pores, which is based on this mechanism, results in a variety of disadvantages in regard to their application. Sound deadening boards are easily soiled, are not washable, absorb water, oil or other liquids and then become ineffective, they cannot be painted over, etc. Since sound deadening boards are also thermal insulation boards, water of condensation often forms within the boards and then drips (ceilings over swimming pools). Attempts are made to avoid these disadvantages by covering the visible side of the boards with thin sheets of plastic. This, however, impairs the penetration of the sound into the sound deadening substance. The covering reflects part of the sound, and the higher the frequency is, and the heavier the covering is, and the more tightly it is applied to the sound deadening substance, the more the sound is reflected. The mechanical strength requirement of the plastic coverings also results in unsatisfactory compromises. This system has advantages only for the absorption of lower frequencies, which can be somewhat higher than it is in sound deadening substances which are not covered with plastic due to resonance effects.
Fundamentally, there is a second possibility for the absorption of sound, namely the conversion of energy by the deformation of solid bodies caused by the sound. This knowledge is not new, and has long been used for the absorption of sound transmitted through liquids and solid bodies. In the case of sound transmitted through air this has not been possible hitherto because the high input impedances of solid bodies render the penetration of airborne sound virtually impossible. The input impedance Z of a medium for sound waves is given by the product of the density .delta. and the velocity of sound c: EQU Z = .delta. .sup.. c (1)
Since even the lightest solids, such as plastic foams for example, have a density that is higher than that of air by a factor of at least 10, the input impedance of the solid, Z.sub.F, will therefore be greater by at least a factor of 10, than the wave impedance Z.sub.L of air. The result is a reflection factor r at the surface, which in any case is greater than ##EQU1##
In the case of all closed-cell hard foams, such as polystyrene foam for example, the stiffness of the structure is furthermore so high that the velocity of sound c.sub.F in the solid is substantially greater than the velocity of sound c.sub.L in the air, so that the ratio ##EQU2## is even more unfavorable. Furthermore, in the known hard foams the internal losses are so low that no appreciable sound absorption can be achieved by deformation.
Closed-cell soft foams, especially cross-linked polyethylene foam, offer a better basis from the outset. The internal losses are high, especially in shear deformation, and the structural stiffness is low. The minimum density .delta..sub.F that is achievable at the present time is still about 20 times greater than that of air, but on the other hand the sound velocity c.sub.F is approximately comparable with that of air.
If a foam of this kind, in which Z.sub.F = 20 Z.sub.L, is used as a sound absorber, very little success will be achieved. Mathematically, we have the following for a vertical impingement of sound: ##EQU3## EQU .alpha. = 1 -/r/ .sup.2 = 0.19 (5)
that is, a sound absorption degree .alpha. of only 19%, which in the case of an oblique impingement of sound is slightly greater but still insufficient. It is for this reason that closed-cell foams are not used for sound absorption purposes.